Phage display technology has provided a powerful tool for generating and selecting novel proteins that bind to a ligand, such as an antigen. Using the techniques of phage display allows the generation of large libraries of protein variants that can be rapidly sorted for those sequences that bind to a target antigen with high affinity. Nucleic acids encoding variant polypeptides are fused to a nucleic acid sequence encoding a viral coat protein, such as the gene III protein or the gene VIII protein. Monovalent phage display systems where the nucleic acid sequence encoding the protein or polypeptide is fused to a nucleic acid sequence encoding a portion of the gene III protein have been developed. (Bass, S., Proteins, 8:309 (1990); Lowman and Wells, Methods: A Companion to Methods in Enzymology, 3:205 (1991)). In a monovalent phage display system, the gene fusion is expressed at low levels and wild type gene III proteins are also expressed so that infectivity of the particles is retained. Methods of generating peptide libraries and screening those libraries have been disclosed in many patents (e.g. U.S. Pat. No. 5,723,286, U.S. Pat. No. 5,432,018, U.S. Pat. No. 5,580,717, U.S. Pat. No. 5,427,908 and U.S. Pat. No. 5,498,530).
The demonstration of expression of peptides on the surface of filamentous phage and the expression of functional antibody fragments in the periplasm of E. coli was important in the development of antibody phage display libraries. (Smith et al., Science (1985), 228:1315; Skerra and Pluckthun, Science (1988), 240:1038). Libraries of antibodies or antigen binding polypeptides have been prepared in a number of ways including by altering a single gene by inserting random DNA sequences or by cloning a family of related genes. Methods for displaying antibodies or antigen binding fragments using phage display have been described in U.S. Pat. Nos. 5,750,373, 5,733,743, 5,837,242, 5,969,108, 6,172,197, 5,580,717, and 5,658,727. The library is then screened for expression of antibodies or antigen binding proteins with desired characteristics.
Phage display technology has several advantages over conventional hybridoma and recombinant methods for preparing antibodies with the desired characteristics. This technology allows the development of large libraries of antibodies with diverse sequences in less time and without the use of animals. Preparation of hybridomas or preparation of humanized antibodies can easily require several months of preparation. In addition, since no immunization is required, phage antibody libraries can be generated for antigens which are toxic or have low antigenicity (Hogenboom, Immunotechniques (1988), 4:1-20). Phage antibody libraries can also be used to generate and identify novel human antibodies.
Antibodies have become very useful as therapeutic agents for a wide variety of conditions. For example, humanized antibodies to HER-2, a tumor antigen, are useful in the diagnosis and treatment of cancer. Other antibodies, such as anti-INF-γ antibody, are useful in treating inflammatory conditions such as Crohn's disease. Phage display libraries have been used to generate human antibodies from immunized and non-immunized humans, germ line sequences, or naïve B cell Ig repertories (Barbas & Burton, Trends Biotech (1996), 14:230; Griffiths et al., EMBO J. (1994), 13:3245; Vaughan et al., Nat. Biotech. (1996), 14:309; Winter EP 0368 684 B1). Naïve, or nonimmune, antigen binding libraries have been generated using a variety of lymphoidal tissues. Some of these libraries are commercially available, such as those developed by Cambridge Antibody Technology and Morphosys (Vaughan et al., Nature Biotech 14:309 (1996); Knappik et al., J. Mol. Biol. 296:57 (1999)). However, many of these libraries have limited diversity.
The ability to identify and isolate high affinity antibodies from a phage display library is important in isolating novel human antibodies for therapeutic use. Isolation of high affinity antibodies from a library is traditionally thought to be dependent, at least in part, on the size of the library, the efficiency of production in bacterial cells and the diversity of the library. See, e.g., Knappik et al., J. Mol. Biol. (1999), 296:57. The size of the library is decreased by inefficiency of production due to improper folding of the antibody or antigen binding protein and the presence of stop codons. Expression in bacterial cells can be inhibited if the antibody or antigen binding domain is not properly folded. Expression can be improved by mutating residues in turns at the surface of the variable/constant interface, or at selected CDR residues. (Deng et al., J. Biol. Chem. (1994), 269:9533, Ulrich et al., PNAS (1995), 92:11907-11911; Forsberg et al., J. Biol. Chem. (1997), 272:12430). The sequence of the framework region is a factor in providing for proper folding when antibody phage libraries are produced in bacterial cells.
Generating a diverse library of antibodies or antigen binding proteins is also important to isolation of high affinity antibodies. Libraries with diversification in limited CDRs have been generated using a variety of approaches. See, e.g., Tomlinson, Nature Biotech. (2000), 18:989-994. CDR3 regions are of interest in part because they often are found to participate in antigen binding. CDR3 regions on the heavy chain vary greatly in size, sequence and structural conformation.
Others have also generated diversity by randomizing CDR regions of the variable heavy and light chains using all 20 amino acids at each position. It was thought that using all 20 amino acids would result in a large diversity of sequences of variant antibodies and increase the chance of identifying novel antibodies. (Barbas, PNAS 91:3809 (1994); Yelton, D E, J. Immunology, 155:1994 (1995); Jackson, J. R., J. Immunology, 154:3310 (1995) and Hawkins, R E, J. Mol. Biology, 226:889 (1992)).
There have also been attempts to create diversity by restricting the group of amino acid substitutions in some CDRs to reflect the amino acid distribution in naturally occurring antibodies. See, Garrard & Henner, Gene (1993), 128:103; Knappik et al., J. Mol. Biol. (1999), 296:57. However, these attempts have had varying success and have not been applied in a systematic and quantitative manner. Creating diversity in the CDR regions while minimizing the number of amino acid changes has been a challenge. Furthermore, in some instances, once a first library has been generated according to one set of criteria, it may be desirable to further enhance the diversity of the first library. However, this requires that the first library has sufficient diversity and yet remain sufficiently small in size such that further diversity can be introduced without substantially exceeding practical limitations such as yield, etc.
Some groups have reported theoretical and experimental analyses of the minimum number of amino acid repertoire that is needed for generating proteins. However, these analyses have generally been limited in scope and nature, and substantial skepticism and questions remain regarding the feasibility of generating polypeptides having complex functions using a restricted set of amino acid types. See, e.g., Riddle et al., Nat. Struct. Biol. (1997), 4(10):805-809; Shang et al., Proc. Natl. Acad. Sci. USA (1994), 91:8373-8377; Heinz et al., Proc. Natl. Acad. Sci. USA (1992), 89:3751-3755; Regan & Degrado, Science (1988), 241:976-978; Kamteker et al., Science (1993), 262:1680-1685; Wang & Wang, Nat. Struct. Biol. (1999), 6(11):1033-1038; Xiong et al., Proc. Natl. Acad. Sci. USA (1995), 92:6349-6353; Heinz et al., Proc. Natl. Acad. Sci. USA (1992), 89:3751-3755; Cannata et al., Bioinformatics (2002), 18(8):1102-1108; Davidson et al., Nat. Struct. Biol. (1995), 2(10):856-863; Murphy et al., Prot. Eng. (2000), 13(3):149-152; Brown & Sauer, Proc. Natl. Acad. Sci. USA (1999), 96:1983-1988; Akanuma et al., Proc. Natl. Acad. Sci. (2002), 99(21):13549-13553; Chan, Nat. Struct. Biol. (1999), 6(11):994-996.
Thus, there remains a need to improve methods of generating libraries that comprise functional polypeptides having a sufficient degree of sequence diversity, yet are sufficiently amenable for further manipulations directed at further diversification, high yield expression, etc. The invention described herein meets this need and provides other benefits.